The present invention generally relates to electrochemical cells, i.e., batteries, and, more particularly, to a separator and method of making and assembling a separator in an electrochemical cell.
Conventional alkaline electrochemical cells generally include a steel cylindrical can having a positive electrode, referred to as the cathode, which comprises manganese dioxide as the active material. The electrochemical cell also includes a negative electrode, referred to as the anode, which comprises zinc powder as the active material. In bobbin-type cells, the cathode is typically formed against the interior surface of the steel can, while the anode is generally centrally disposed in a cylindrical cavity formed in the center of the cathode. A separator is located between the anode and the cathode, and an alkaline electrolyte solution containing potassium hydroxide (KOH) simultaneously contacts the anode, the cathode, and the separator. A conductive current collector is commonly inserted into the anode active material, and a seal assembly, which includes a polymeric seal, provides closure to the open end of the steel can to seal the active electrochemical materials in the sealed volume of the can.
In conventional bobbin-type zinc/manganese dioxide alkaline cells, the separator is commonly provided as a multiple layered ion permeable, non-woven fibrous fabric which separates the anode from the cathode. The separator maintains a physical dielectric separation of the positive electrode material and the negative electrode material and allows for the transport of ions between the electrode materials. In addition, the separator acts as a wicking medium for KOH solution and as a collar for preventing the anode gel from falling out. Examples of conventional separator materials include two or three layers of paper, which results in a total wet separator thickness in the range of about 11-18 mils. Conventional separators are usually formed either by preforming the separator material into a cup-shaped basket that is subsequently inserted into the cathode during assembly, or forming a basket during cell assembly by inserting into the cathode cavity multiple rectangular sheets of separator material angularly rotated ninety degrees relative to each other. The conventional preformed separators are typically made up of a sheet of non-woven fabric rolled into a cylindrical shape that conforms to the inside walls of the cathode and has a closed bottom end. According to another approach, the closed end is provided by inserting a dielectric seal in the form of a plug in the bottom end of the steel can and inserting a convolute cylindrical separator up against the plug.
The conventional separator employs a fibrous porous paper that generally requires multiple overlapping layers in order to prevent electrical conduction between the anode and the cathode. The use of a single layer of paper for a separator generally suffers from openings that are present in the porous paper which may allow a conductive path to be formed between the anode and the cathode. It is also possible that the graphite in the cathode may penetrate the separator to form a conductive path with the anode, thereby causing cell shorting. Further, the formation of zinc oxide within the pores of the paper separator may also form an electrically conductive path that causes cell shorting and leads to premature discharge.
The use of multiple paper layers increases the volume consumed by the separator, thereby leaving less volume for active electrochemical materials. In addition, a thicker separator generally increases the ionic resistance which results in a reduced ion permeation and limits the high rate discharge performance. Many conventional separators do not minimize the amount of separator material that is disposed in the cell, which results in reduced volume available for electrochemically active materials. Accordingly, it is therefore desirable to provide for a separator for use in electrochemical cells that efficiently separates the positive and negative electrodes, while minimizing the amount of material required to separate the electrodes, to thereby enhance ion permeation and maximize the volume available for electrochemically active materials.
The present invention improves the separation of the electrodes in an electrochemical cell with an enhanced separator, a method of making the separator, and a method of assembling the separator in an electrochemical cell. To achieve these and other advantages, and in accordance with the purpose of the invention as embodied and described herein, one aspect of the present invention provides for a separator for use in an electrochemical cell for separating a positive electrode and a negative electrode. The separator comprises a porous substrate capable of absorbing alkaline solution. The separator also has a polymer solution applied to the porous substrate in the presence of the alkaline solution. The polymer solution coagulates to form a semi-solid impregnation in the porous substrate and/or a semi-solid coating on the porous substrate that prevents electrical shorting and allows ion permeation through the separator. The resultant separator is capable of achieving reduced volume consumption and enhanced ion permeation.
According to another aspect of the present invention, a method of forming a separator is provided for use in an electrochemical cell for separating a positive electrode from a negative electrode. The method comprises the steps of providing a porous substrate and applying a coagulating agent to the porous substrate. The method further includes the steps of applying a liquid polymer solution to the porous substrate and allowing the polymer solution to coagulate in the presence of the coagulating agent to form a semi-solid material.
According to other aspects of the present invention, an electrochemical cell and a method of assembling an electrochemical cell are provided. The method includes the steps of providing a container having a bottom end and a top end and upstanding walls disposed therebetween, disposing a positive electrode in the container, and disposing a negative electrode in the container. The method also includes providing a porous substrate, forming the porous substrate material into a separator, and applying electrolyte solution to the separator. The method further includes the steps of applying a liquid polymer solution to the porous substrate in the presence of a coagulating agent so that the polymer solution coagulates to form a semi-solid material. The separator is disposed between the positive electrode and the negative electrode, preferably prior to applying the liquid polymer solution.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.